It is known in the art to use precious or semiprecious metals as electrode tips in spark plugs for prolonged life. For example, in the patent to Heller 2,296,033 the precious metal tip is welded to a body of much less expensive metal forming the center electrode and itself acts as the firing tip. This is true in a number of earlier designs. It has also been proposed to use tungsten or a tungsten alloy in such a construction as, for example, in the patent to Hensel 2,391,455. However, tungsten and tungsten alloys corrode rapidly in some modern internal combustion engines and any construction in which the sides of the tungsten tip are not protected show very low electrode life in certain services.
It is also known in the art, by the patent to Hensel 2,391,456, that the end of an electrode may be capped with a sheath of platinum or the like to form a long-wearing electrode where the core of the electrode is composed of molybdenum or a molybdenum alloy having high heat conductivity and a coefficient of thermal expansion substantially the same as that of the cap or sheath. The most common constructions have been to use a rivet of platinum seated in the bore of the insulator tip or into the bore of a split nickel sleeve as in the patent to Bretsch 3,256,457. The rivet is held in place by casting silver metal around the upper portion of the rivet head.
In practice the rivet construction requires a large amount of precious metal which contributes nothing useful to the sparking life of the electrode. A very large portion of the precious metal is wasted as a support rod. Also in practice it has been found difficult to attach the rivet securely for long periods of operation in the corrosive atmosphere of the combustion chamber.
In all of the prior art constructions the quantity of precious metal used is so high that, under modern conditions the price of the spark plug becomes excessive and the limited recourses of the precious metals are largely wasted.
In known spark plugs, the center electrode protrudes from its supporting insulator and deposits on the insulator surface are deliberately excluded from the firing process. It has been found that under some circumstances these deposits can actually be used in a beneficial manner to extend the life of the spark plug.